"I can easily see still in my mind’s-eye the beautiful clusters of these berries as they appeared to me..., when I came upon an undiscovered bed of them... – the rich clusters drooping in the shade there and bluing all the ground" -- Thoreau

Cool! Though the fact that CRISM doesn't spectroscopically see any water associated with the dark features makes me somewhat skeptical. If they are made by water, why aren't they wet? I hope they keep scanning with CRISM in the hopes of catching one in the act- maybe it's a matter of looking at the right time of day.

Or maybe they are dust avalanches that happen to be triggered by summer winds, though in that case it's surprising that they advance downslope so slowly and gradually. Hmmm....

That's weird CRISM does not see anything. They argue that flows are much smaller than the CRISM pixel scale (18m). That's true for a single flow, which width scales between 0.5 and 5m, but some HiRISE pictures show 60m*60m areas covered by this dark flows. So CRISM shoud see them. And a mixture of water and salt does not change anything : there is still a strong signature of water. I'm very skeptical...

That's weird CRISM does not see anything. They argue that flows are much smaller than the CRISM pixel scale (18m). That's true for a single flow, which width scales between 0.5 and 5m, but some HiRISE pictures show 60m*60m areas covered by this dark flows. So CRISM shoud see them. And a mixture of water and salt does not change anything : there is still a strong signature of water. I'm very skeptical...

the water will boil off (evaporate) quickly under the low pressure. Suspect there would be some kind of evaporite (salt) left over. But for direct detection of water CRISM would almost need to catch it in the act.

do any of the currently active orbiters have an instrument that can determine atmospheric humidity levels with high spatial resolution?

Mars Climate Sounder onboard MRO can detect water vapor (and thus you can calculate humidity as it detects temperature also).

But it probably wouldn't work for this scenario. It's not ideal for detecting these values in the lowest layers in the atmosphere and whatever water comes out of these features probably won't significantly effect the water vapor concentration at higher altitudes.

Amusingly, it's similar images of water that make me wonder if the albedo changes simply have to do with the clumpiness of the dust in the streaks. There's activity in snowpack called either sloughing/sluffing. Basically by altering only the texture of the snow, it creates streaks like the ones found in the dust on Mars:

I guess this sort of thing is more common with SAR... but you can see it in plain old visible light. The whole slope is bright white snow. Up close the color of the sluffs is no different. They're just reflecting the sky back to the camera differently.

Like snow, the dust probably falls in dry and sparsely packed, and clumps up more when something starts it rolling.

I wonder if the streaks are just clumpier dust on airfall dust. They do need a trigger, though. That could be a tiny bit of water starting a basically dry sluff.

What would be the correlation to seasonal changes for the reflectivity of dust/soil? It seemed as if that was a major component to the liquid water hypothesis.

Whatever triggers the streaks is seasonal, sure. I'm open to the trigger being water.. Something changes the properties of one small section of dust, and a cascade starts. I just think a sluff of dry dust could also explain the apparent albedo change, if dry dust rolling downslope is simply clumpier than dust that fell from the atmosphere.

They slowly fade due to constant dust falling from the atmosphere. I think with any explanation for causing their appearance, that's the reason they disappear, right?

I actually don't think dry processes can explain it all. Sure, a few exfoliating pieces of gravel could drop onto dust slopes that are at their maximal steepness and start dry cascades... but it seems like there's way too much action going on, so of it triggered away from any rock faces. The real question in my mind seeing the apparent albedo change... is there necessarily a change in composition, or is it just a change in texture? It looks more wet... it may just be more rough. (Or ruff, if I stick with the phonetic version of slough.)

Of course i hope its water, but somehow wouldnt we expect dampened dust to be cohesive enough to clump together and change properties enough to over time alter the topography subtly but enough that we shoudl see some sort of evidence of that after repeated episodes over eons? With the analogous snow scenario posted above you wouldnt get any clumping or change of cohesion so that seems more persuasive. thanks for posting that.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted.
Do not reproduce without permission. Read
here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the
individual posters and do not necessarily reflect the opinions
of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer
UnmannedSpaceflight.com moderation team is wholly independent
of The Planetary Society. The Planetary Society has no influence
over decisions made by the UnmannedSpaceflight.com moderators.

SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society
and is funded by donations from visitors and members. Help keep
this forum up and running by contributing
here.